The objective of this study is the continued long term assessment of the host-parasite interaction among 100-200 adult, female patients with clinical infections of the urinary tract of varying frequency and severity. This study is designed to increase our knowledge of the pathogenesis and natural history of patients with single or multiple recurrences of infection and of their potential relationship to the later development of arterial hypertension and/or chronic renal failure. Most of these patients have now been followed carefully for 7-11 years. The patients were initially selected on the basis of a history of active and/or past infections of the urinary tract. They were required to have a normal intravenous pyelogram and no evidence of hypertension or other diseases known to predispose to chronic renal failure. During the past study periods over 400 episodes of infection have been confirmed. The clinical syndromes have been characterized, and a number of measurements of host response made including complete blood counts and urinalysis, serum creatinines, host antibody response to the infecting strains of E. coli and repeat intravenous pyelograms and renal concentrating tests following vasopressin stimulation in selected patients. In addition to the routine diagnostic characterization of the infecting strains those due to E. coli have been serologically subclassified to better define recurrent disease. Bibliographic references: Vosti, K.L.: Prevention of recurrent urinary tract infections by prophylactic antibiotics after sexual intercourse. J.A.M.A. 231: 934-940, 1975.